1. Field of the Invention
The present invention relates to an optical transmission line suitable for long-haul transmission and large-capacity transmission of signal light.
2. Related Background Art
Optical transmission lines constituted by optical fibers have conventionally been used for large-capacity high-speed communications such as image communications, and long-haul communications such as international communications, while there are demands for larger capacity and longer distance. In order to achieve larger capacity wavelength division multiplex (WDM) transmission is employed, in which a plurality of wavelengths of signal light are multiplexed. To increase transmission distance, on the other hand, intervals between repeaters in optical fiber networks are increased, while the optical amplification factor in each repeater is enhanced, thus allowing the signal light outputted from the repeater to increase its power such that it reaches the repeater in the next stage with a sufficient power.
Meanwhile, it has been known in general that, when light propagates through a medium, a nonlinear optical phenomenon occurs. This nonlinear optical phenomenon includes, for example, self-phase modulation (SPM), four-wave mixing (FWM), cross-phase modulation (XPM), nonlinear scattering, or the like, and becomes greater as the optical energy density per unit volume is higher in the medium. Accordingly, when the optical amplification factor in a repeater is enhanced in order to cover a longer distance in an optical fiber transmission line, such a nonlinear optical phenomenon would increase as well. When a nonlinear optical phenomenon occurs in the optical transmission line, signal light deteriorates, whereby reception errors are likely to occur in its receiving station.
Hence, several proposals have been made in order to solve such problems. For example, Japanese Patent Application Laid-Open No. 6-85757 discloses a technique in which two optical fibers having effective areas different from each other and wavelength dispersions different from each other are cascaded to each other so as to constitute an optical transmission line, thereby reducing the deterioration in its optical transmission characteristics caused by the occurrence of nonlinear optical phenomena and the wavelength dispersion. Also, Japanese Patent Application Laid-Open No. 8-248251 discloses a technique which uses a dispersion-shifted optical fiber having an effective area .gtoreq.Nm.sup.2, which is greater than that of the usual dispersion-shifted optical fiber, thereby reducing the deterioration in its optical transmission characteristics caused by the occurrence of nonlinear optical phenomena. Further, Japanese Patent Application Laid-Open No. 8-304655 discloses a technique which uses a dispersion-shifted optical fiber having a wavelength dispersion slope of (0.05 ps/nm.sup.2 /km) smaller than that of the usual dispersion-shifted optical fiber, thereby reducing the deterioration in its optical transmission characteristics caused by the occurrence of nonlinear optical phenomena.